This invention relates in general to two-way radio messaging systems, and particularly to a radio communication terminal for optimizing transmission of messages to selective call transceivers and method therefor.
Generally, two-way radio messaging systems (hereinafter referred to as messaging systems) comprise a radio communication terminal and a plurality of portable SCTs (Selective Call Transceivers) for communicating messages thereto. The radio communication terminal uses frequency reuse methods for maximizing message capacity to portable SCTs. Frequency reuse is accomplished by assigning a frequency pattern to a cluster of communication zones, and repeating the frequency pattern across other clusters. When the number of communication zones in a cluster is decreased, frequency reuse is increased, thereby increasing system capacity. As a means for tracking SCTs in the messaging system, each SCT is programmed to request registration from the radio communication terminal each time the SCT enters a new zone. By knowing the location of each SCT, the messaging system is able to optimally use the frequency reuse plan.
Generally, when a message is transmitted by the messaging system to a SCT, the SCT responds with an acknowledgement indicating to the messaging system that the message has been received. However, it is not uncommon for the radio communication terminal to not receive a return acknowledgement from the SCT. This is because portable battery-operated SCTs have very little power for transmitting inbound messages over long distances, and depending on the distribution of receivers of the radio communication terminal, it is possible that a SCT will be situated in a location where receiver reception is poor. In contrast, the probability of outbound messages being received by SCTs from the radio communication terminal is substantially higher, because the power used to transmit messages to the SCT is comparably high.
To account for the situation where the radio communication terminal fails to receive an acknowledgement message from a SCT, prior art systems employ what is generally termed in common parlance as a fallback delivery strategy. Under this strategy, the messaging system reverts to a one-way communication strategy in which the radio communication terminal retransmits the message to the non-responding SCT over a pre-selected number of communication zones. Although message delivery is not guaranteed under this strategy, the probability of the SCT receiving the message is increased as a result of broadcasting the message over several communication zones.
Although this strategy is better than not retransmitting the message at all, the approach of unintelligently retransmitting the message over a pre-selected number of communication zones is not an optimal approach for locating a non-responsive SCT. Accordingly, a need exists in the prior art for improving the strategy of locating non-responsive SCTs.